1. Field of Invention
This invention relates generally to discrete multitone transmission systems, and more particularly to peak-to-average power ratio (PAR) control in such systems. Embodiments of the invention provide methods and apparatus for generating discrete multitone signals with reduced PAR.
2. Description of the Related Art
In transmission systems employing multicarrier modulation, the transmission channel is generally divided into N narrowband subchannels with centre frequencies equally spaced across the usable frequency band, and the input data to be communicated over the transmission channel is used to modulate the N carriers. The basic principles of such systems will be described with reference to FIGS. 1a to 1c of the accompanying drawings. Referring to FIG. 1a, the input data is grouped into blocks of S bits at a block, or “symbol”, rate of fs. In each sample period, or “symbol period”, T=1/fs, the block of S bits is partitioned into groups of mk bits, where k=0 to N−1, and each mk is typically 2 to 8 bits. The groups of mk bits are then used to modulate N carriers fk spaced Δf apart across the usable frequency band. Specifically, as illustrated schematically in FIG. 1b, each group of mk bits is used to index a frequency domain modulation symbol Xk which is taken in general from a QAM (Quadrature Amplitude Modulation) constellation with Mk=2mk complex points. FIG. 1c illustrates such a constellation for mk=4, so that 16 complex points are provided for the 16 possible values of the 4-bit sequence mk. Discrete multitone (DMT) modulation is a particular form of multicarrier modulation in which an inverse discrete Fourier transform (IDFT) is employed for signal modulation:
                                          x            n                    =                                                    ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  k                                ⁢                                  ⅇ                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                    π                    ⁢                                                                                  ⁢                                          nk                      /                      N                                                                      ⁢                                                                  ⁢                n                                      =            0                          ,        1        ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            Thus, in each symbol period T, the N carriers are modulated as indicated in FIG. 1b by performing an IDFT of the N modulation symbols Xk to generate a DMT symbol (x0, x1, . . . xN−1). Since the phases of the addends in equation (1) can build up constructively, the peak value of the signal xn, relative to its average energy becomes an issue of concern for the realization of DMT-based transmission systems. The peak-to-average power ratio of such a system is defined by:PAR=10 log10(P2/σx2)(dB)  (2)
In equation (2), P denotes the peak value of the signal sequence at the output of the IDFT, and σx2=E{xn2} with E denoting statistical expectation. For a well designed system, the probability of the PAR exceeding a threshold value defined by implementation constraints should be low. DMT systems with low PAR values allow efficient design of the signal converters, amplifiers, and the line driving and receiving circuits. By way of example, DMT is currently used in Digital Audio Broadcast and ADSL (Asymmetric Digital Subscriber Line), and is also proposed for VDSL (Very High Data Rate Digital Subscriber Line).
Schemes for providing PAR control in DMT systems are known in the art. Most of the known schemes involve transmitting side information, defining a transformation of the signal performed for PAR control, along with the data signal itself to enable the receiver to correctly demodulate the signal. With these schemes there is inherently a data rate loss due to the need to transmit the side information. ITU-T, Study Group 15, Temporary Document NF-83, Nice, France 11-14 May 1998 (A New Approach to PAR Control in DMT Systems) discloses a system which avoids data rate loss through use of expanded (redundant) QAM constellations. In this system, the QAM constellations are expanded beyond the minimum size necessary to support the required data rate on each subchannel. Specifically, for each point in the original minimum energy constellation, a number of equivalent points are defined by a modulo operation, the original and modulo-equivalent points constituting an “equivalence class” of points which represent the same input data. The input data is mapped to a particular equivalence class, and a PAR reduction algorithm then selects a particular signal point in that class, choosing the point which reduces the time domain peak amplitude. ANSI T1E1.4, Contribution 98-173, 4 Jun. 1998 (PAR Reduction with Minimal or Zero Bandwidth Loss and Low Complexity) also discloses a system which employs expanded constellations for PAR control. The particular technique discussed in detail in this document again involves defining equivalents to the standard minimum energy constellation points by a modulo operation. The peak value of the DMT symbol obtained using the minimum energy constellation points is first determined, and then an alternative, equivalent point is selected according to certain criteria and the DMT symbol is updated. This process is repeated until the desired PAR is achieved or certain performance limits are reached.
While the systems disclosed in the two documents referenced above can avoid data rate loss through use of expanded constellations, the penalty is an increase in the average transmit power and the additional complexity introduced by the modulo operations and PAR reduction algorithms.